1. Field of the Invention
The invention relates to the field of optical systems, and in particular, to an fiber optic system that mitigates polarization mode dispersion (PMD) on optical signals.
2. Description of the Prior Art
A fiber optic system transfers information by transmitting light over a fiber. The fiber optic system performance depends on how the light propagates through the fiber and is measured in terms of bit error rates (BER). Light propagation can be impaired for a number of reasons such as attenuation, fiber non-linearities, and dispersion. The demand for high-performance fiber optic systems means the effects of propagation impairments must be minimized.
Dispersion refers to the smearing or spreading out of a pulse as it propagates through a fiber. The pulse eventually spreads out enough that it overlaps with neighboring pulses causing intersymbol interference (ISI). ISI sharply increases the BER of the system. One kind of dispersion is Polarization Mode Dispersion (PMD). PMD is a key limitation to the high-speed optical systems. PMD is measured like a vector quantity, where a Differential Group Delay (DGD) is the magnitude of the vector and Principal States of Polarization (PSP) are the direction. There are two PSPs associated with PMD. The two PSPs propagate at slightly different velocities with the distribution of signal power varying with time. The different velocities cause a pulse to broaden causing PMD. PMD is a time varying stochastic effect. PMD varies in time with ambient temperature, fiber movement, and mechanical stress on the fibers. Compensating for PMD can be difficult because of the time varying nature and randomness of PMD. The amount of PMD in an optical signal must be kept at a minimum in order to keep a low BER within the fiber optic system.
FIG. 1 shows a fiber optic system 100 in the prior art that compensates for PMD in optical signals. Fiber optic system 100 includes a transmitter 102, a PMD compensation system 104, and a receiver 106. PMD compensation system 104 includes a polarization controller 120 and a feedback circuit 122. Feedback circuit 122 is comprised of a Degree-of-Polarization (DOP) analyzer 130 and a processor 132. Transmitter 102 is coupled to polarization controller 120 by a first fiber. Polarization controller 120 is coupled to DOP analyzer 130, and receiver 106 by a second fiber. DOP analyzer 130 is coupled to processor 132. Processor 132 is coupled to polarization controller 120.
In operation, transmitter 102 transmits a first optical signal to polarization controller 120 over the first fiber. Polarization controller 120 maximizes the DOP in the first optical signal to generate a second optical signal. An example of polarization controller 120 is a “fiber squeezer”, which is a device that squeezes a fiber and changes the DOP in a signal by adding stress to the fiber. Polarization controller 120 transfers the second optical signal to DOP analyzer 130, and receiver 106 over the second fiber.
Feedback circuit 122 monitors the DOP of the second optical signal to ensure that the second optical signal has a maximum DOP. DOP analyzer 130 receives the second optical signal from polarization controller 120. DOP analyzer 130 measures the DOP in the second optical signal to generate a corresponding DOP measurement signal. DOP analyzer 130 transfers the DOP measurement signal to processor 132. Processor 132 processes the DOP measurement signal to determine whether the DOP in the second optical signal is at a maximum. Processor 132 generates and transfers a control signal to polarization controller 120. If the DOP in the second optical signal is at a maximum, the control signal does not alter the operation of polarization controller 120. If the DOP in the second optical signal is not at a maximum, the control signal changes the operation of polarization controller 120 to increase the DOP of the second optical signal.
Fiber optic systems need to mitigate PMD in optical signals in order to provide high-speed communications. A problem with fiber optic system 100 is that PMD compensation system 104 is complex and expensive. PMD system 104 uses feedback circuit 122 to monitor and correct system performance, and feedback circuit 122 adds complexity and cost to fiber optic system 100.